The present invention relates to an orthogonal frequency division multiplexing (OFDM) receiver device in communication systems utilizing an orthogonal frequency division multiplexing system.
Recent communication systems are required to transmit large capacity data such as video information or the like as the digital value not only with wire transmission system but also with radio transmission system. In this case, it is essential to introduce not only the Phase Shift Keying system to modulate the phase with the information by utilizing difference of phase such as BPSK and QPSK or the like but also the Quadrature Amplitude Modulation system to modulate the phase and amplitude with the information by utilizing differences of phase and amplitude such as 16 QAM and 64 QAM or the like. The signal modulated with the QAM method such as 16 QAM and 64 QAM is transmitted from a transmitter. The transmitted signals is then received by a receiver device through the transmission path, and demodulated to the original data through the synchronous detection.
In this case, when the wired transmission path is used, any problem does not occur. However, when the radio or wireless transmission link is established, a large distortion is generated in both amplitude and phase of the received signal due to poor transmission path caused by, for example, fading. Therefore, as a method of alleviating distortion with the transmission path, it is proposed to hold the same known pilot signal (known signal) in both transmitter and receiver device. Such a pilot signal is transmitted from the transmitter and the transmission path response is determined by using the pilot signal received with the receiver device and the pilot signal held in the receiver device. Then the transmission path is estimated by interpolating the transmission path response in order to compensate for both amplitude and phase of the received data signal (information signal).
For example, JP-A-11-163822 teaches a system that is utilized to the communication system signal using the OFDM system to compensate for distortion in both amplitude and phase of the data signal included in the received OFDM signal.
The above prior art discloses an OFDM receiver device which is used in the digital broadcasting system using the ground wave. Therefore, it is a precondition that the format of OFDM signal in the European DVB-T system or the like as illustrated in FIG. 18 is used. In FIG. 18, the vertical direction indicates the time (symbol), while the horizontal direction indicates frequency (carrier). Moreover, white circles in the same figure define data symbols (data signals), while the black circles define the pilot symbols (pilot signals). The pilot symbol is transmitted in every 12 carrier frequencies and is cyclically allocated so that the same sub-carrier frequency is attained after the four symbols. The OFDM receiver device disclosed in the above prior art compensates for distortion of the amplitude and phase of the received data signal and performs equalization on the frequency axis for the OFDM signal of the format illustrated in FIG. 18.
As the format of OFDM signal, there is proposed the OFDM signal format for MMAC (Multimedia Mobile Access Communication) in addition to the format for the above ground digital broadcast. In this OFDM signal format, as illustrated in FIG. 19, the data signals (white circles in the figure) of 0 to 4, 5 to 17, 18 to 29, 30 to 42, 43 to 47 are allocated in the frequency direction. The pilot signals (black circles in the figure) are also dispersed among such data signals. Moreover, these allocations are identical in the time direction.
In the case of this OFDM signal format, the data signals of 0 to 4 are allocated in the side of frequency lower than the pilot signal in the lowest frequency side among the four pilot signals. The data signals of 43 to 47 are allocated in the side of frequency higher than the pilot signal in the highest frequency side among four pilot signals. Since the OFDM signal format of FIG. 18 is different from that of FIG. 19 as explained above, it is impossible to adequately compensate for both amplitude and phase of the received data signal for the OFDM signal format for MMAC as illustrated in FIG. 19 in the OFDM receiver device disclosed in the above prior art (JP-A-11-163822).